1. Field of the Invention
This invention relates in general to molded case circuit breakers and more particularly to molded case circuit breakers employing thermal and magnetic trip assemblies.
2. Description of the Prior Art
Small molded case automatic circuit breakers are widely used in commercial, industrial and residential installations to provide protection against over-current conditions. In order to perform reliably these circuit breakers must be accurately calibrated to interrupt the flow of current at the desired overload level. They must remain in calibration over long periods of time and over wide extremes of temperatures occurring in the operating environment. Since the calibration of these circuit breakers is dependent on physical dimensions between the various components it is important that these dimensions remain constant over the expected extremes of temperature. One method of insuring dimensional stability over temperature extremes is to mount critical components of the trip assembly upon a metal frame enclosed within the molded insulating case. This method is employed in the circuit breaker described in U.S. Pat. Nos. 3,088,008 and 3,110,786 issued to Francis L. Gelzheiser and assigned to the assignee of the present invention. If these critical components can be supported directly by the molded case and means can be provided to adequately adjust for mechanical distortion of the case, the metal frame can be eliminated, thereby reducing the cost of the circuit breaker.
Elimination of the frame is possible if the insulating case is molded of material having a low thermal coefficient of expansion, such as phenolic. However, phenolic exhibits undesirable characteristics under arcing conditions produced by separating contacts, thereby requiring metallic arc chutes to shield the phenolic from the arc.
A circuit breaker with an insulating case molded from urea would not require the use of arc chutes, for urea does not exhibit the undesirable characteristics of phenolic material under arcing conditions. In fact, urea will release an arc-extinguishing gas when subjected to the high temperatures produced by an arc. However, urea is not as mechanically stable under varying temperature conditions as is phenolic, and is therefore unsuitable for those applications requiring a circuit breaker with accurate calibration. It would be desirable to produce a molded case circuit breaker employing urea or other low-cost material as the case material wherein the calibration of the breaker would not be affected by thermal expansion or other physical distortion of the case.